1. Field of the Invention
The present invention relates to a light addressing display device which may be expected to apply to AV equipment or OA equipment like a computer requesting a TV image display and an optical image information processing apparatus.
2. Description of the Related Art
When transmitting a driving signal on an electric wire, the implementation of a large display device or a high-definition display device is made impossible, because the signal waveform is delayed by resistance or capacitance on wires. To solve this disadvantage, a light scan type display device may be used for transmitting a light signal as a scan signal. That is, in the case of using a ray of light as a scan signal, unlike the case of using an electric signal, no delay of the signal waveform takes place without being influenced by the resistance or capacitance on wires.
To solve these problems, there has been developed a light addressing type display device which is arranged to transmit a driving signal with light (Japanese Patent Lying-open Nos. Hei 2-89029 and 1-173016, Japanese Patent Application No. Hei 3-263947 and the like).
FIGS. 17 and 18 show a light scan type active-matrix system liquid crystal display device which the applicants of the present invention proposed in the Japanese Patent Application No. Hei 3-263947.
This liquid crystal display device is arranged to have a pair of a base substrate 121 and an opposed substrate 122 and a liquid crystal 108 laid between the pair of substrates 121 and 122. The base substrate 121 has a plurality of light waveguides Y1, Y2, . . . , Yn on the glass substrate 105a, those light waveguides being ranged horizontally. On those light waveguides, a plurality of signal wires X1, X2, . . . , Xm are ranged vertically in a manner to be crossed with the light waveguides. In each section defined by crossing the light waveguide Y (Y is used for describing one light waveguide. Likewise, X is used for describing one signal wire) and the signal wire X, the pixel electrodes 104, 104, . . . are formed on the glass substrate 105a in a manner to bury the sections. Part of each pixel electrode is formed in a manner to expand over one of the light waveguides Y for defining this section.
At a crosspoint between this light waveguide Y and the signal wire X, there is provided a light switching element 103 made of a photoconductive layer on the light waveguide Y. This light switching element 103 is overlapped with part of the pixel electrode 104 extending over the light waveguide Y. Hence, the light switching element 103 is vertically laid between the overlapped portion of the pixel electrode 104 and the signal wire X. And, an orientation film 109a is formed over all the pixel area of the substrate in a manner to cover them.
The photoconductive layer for forming the light switching element 103 is formed by means of a plasma CVD (Chemical Vapor Deposition) technique. The photoconductive layer uses hydrogenated amorphous silicon (a-Si:H) which may be formed on a large area at low temperature. Since the photoconductive layer needs to raise its impedance in a dark state where no light is applied, intrinsic hydrogenated amorphous silicon (i-a-Si:H) is used.
On the outside of the glass substrate 105a, a light cut-off layer 110a is provided at a location corresponding to each light switching element 103. This light cut-off layer 110a operates to prevent outside light from the outer surface of the glass substrate 105a from being incident on the light switching element 103.
On the inside of the glass substrate 105b composing an opposed substrate 122, a transparent electrode 106 is formed over all the pixel area. On the transparent electrode 106, a light cut-off layer 110b is formed at a location for each light switching element 103 on the base substrate 105a. This light cut-off layer 110b serves to prevent the outside light from the outer surface of the glass substrate 105b from being incident on the light switching element 103. An orientation film 109b is formed to cover the transparent electrode 106.
Such a light scan type liquid crystal display device operates as follows. When the luminous element array 101 and the micro lens array 102 apply a ray of light to the switching element 103 through the light waveguide Y, the light switching element 103 lowers its impedance, when a signal voltage is applied so that the signal wire X may be electrically connected with the pixel electrode 104. When no light is applied, the light switching element 103 raises its impedance, so that the signal wire X may be electrically insulated from the pixel electrode 104. That is, the optical scan type liquid crystal display device uses a ray of light as a scan signal. According to the change of impedance of the light switching element 103, the scan signal is switched on and off for selectively driving the pixel electrode 104.
By the way, the impedance of the photoconductive layer in the dark state where no light is applied is greatly dominated by injecting carriers (secondary current) from the electrode. Hence, in the electrode arrangement having an Ohmic junction, the impedance of the photoconductive layer in the dark state is not allowed to rise to a sufficiently large value. This also results in being unable to disadvantageously obtain such a sufficient on/off ratio of the impedance as meeting a request for sequentially driving the liquid crystal line by line.
On the other hand, heretofore, the use of a diode structure for the light switching element 103 makes it possible to develop a liquid crystal display device which offers an improved switching characteristic. The diode used therefore may include a pin diode, a Schottky diode, a MIS diode and the like. In general, the diode indicates a current to voltage characteristic as shown in FIG. 19 in the dark state where no light is applied. As shown by a broken line of FIG. 19, the current value I keeps a value close to "0" at high impedance in the reversely biased state. This is because the energy barrier serves to inhibit injection of carriers. When light is applied, the similar injection of carriers is inhibited, while as shown by a real line of FIG. 19, the influence of carriers generated by the light becomes more dominant as the light intensity increases (a thin line of FIG. 19 shows when the light application is small and a thick line of FIG. 19 shows when the light application is large). Hence, even in the reversely biased state, the low impedance can be kept. As such, by arranging the light switching element 103 to have a diode structure and using the reversely biased characteristic, it is possible to increase the change of impedance (on/off ratio) against a bright/dark state. These diode characteristics are described in detail in "Basics of Semiconductor Device (Ohm edition, Ltd.)", for example.
To keep the reversely biased state, however, it is necessary to constantly keep feeding of a single-biased data signal to the light switching element 103. In this case, the single-biased (d.c. components) signal is being applied to the liquid crystal served as a display medium when driving the device. When the single-biased signal is applied to the liquid crystal, decomposition of liquid crystal molecules or fitting of impurity on the electrode is more likely to take place. This is disadvantageous in light of reliability. Therefore, the display device having liquid crystal as a display medium should be driven by an a.c. waveform signal containing no d.c. components. It means that the display device requires the light switching element 103 indicating a symmetric characteristic against voltage.